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Koskela, Minna M.; Dahlstrom, Kaethe M.; Goni, Guillermina; Lehtimaki, Nina; Nurmi, Markus; Velazquez-Campoy, Adrian; Hanke, Guy; Boelter, Bettina; Salminen, Tiina A.; Medina, Milagros; Mulo, Paula (2018): Arabidopsis FNRL protein is an NADPH-dependent chloroplast oxidoreductase resembling bacterial ferredoxin-NADP(+) reductases. In: Physiologia Plantarum, Vol. 162, No. 2: pp. 177-190
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Abstract

Plastidic ferredoxin-NADP(+) oxidoreductases (FNRs;EC:1.18.1.2) together with bacterial type FNRs (FPRs) form the plant-type FNR family. Members of this group contain a two-domain scaffold that forms the basis of an extended superfamily of flavin adenine dinucleotide (FAD) dependent oxidoreductases. In this study, we show that the Arabidopsis thaliana At1g15140 [Ferredoxin-NADP(+) oxidoreductase-like (FNRL)] is an FAD-containing NADPH dependent oxidoreductase present in the chloroplast stroma. Determination of the kinetic parameters using the DCPIP NADPH-dependent diaphorase assay revealed that the reaction catalysed by a recombinant FNRL protein followed a saturation Michaelis-Menten profile on the NADPH concentration with k(cat)=3.2 +/- 0.2s(-1), K-m(NADPH)=1.6 +/- 0.3M and k(cat)/K-m(NADPH)=2.0 +/- 0.4M(-1)s(-1). Biochemical assays suggested that FNRL is not likely to interact with Arabidopsis ferredoxin 1, which is supported by the sequence analysis implying that the known Fd-binding residues in plastidic FNRs differ from those of FNRL. In addition, based on structural modelling FNRL has an FAD-binding N-terminal domain built from a six-stranded -sheet and one -helix, and a C-terminal NADP(+)-binding / domain with a five-stranded -sheet with a pair of -helices on each side. The FAD-binding site is highly hydrophobic and predicted to bind FAD in a bent conformation typically seen in bacterial FPRs.